simple solution to einstein's riddle (also known as zebra puzzle)

einstein.rb

require "csp"

problem = CSP::Solver::Problem.new

houses = %w(blue green red white yellow)
nation = %w(british danish german norwegian swedish)
drinks = %w(beer coffee milk tea water)
cigars = %w(master dunhill pall prince blend)
pet    = %w(cat bird dog fish horse)

entities = [houses, nation, drinks, cigars, pet]

entities.each do |e|
  problem.vars e, 1..5
  problem.all_different(e)
end

problem.constrain("british", "red"){ |b, m| b == m }
problem.constrain("swedish", "dog"){ |s, d| s == d }
problem.constrain("danish", "tea"){ |d, t| d == t }
problem.constrain("green", "white"){ |g, w| g == w - 1 }
problem.constrain("green", "coffee"){ |g, c| g == c }
problem.constrain("pall", "bird"){ |p, b| p == b }
problem.constrain("yellow", "dunhill"){ |y, d| y == d }
problem.assign "milk" => 3
problem.assign "norwegian" => 1
problem.constrain("blend", "cat"){ |b, c| (b - c).abs == 1 }
problem.constrain("horse", "dunhill"){ |h, d| (h - d).abs == 1 }
problem.constrain("master", "beer"){ |m, b| m == b }
problem.constrain("german", "prince"){ |g, p| g == p }
problem.constrain("norwegian", "blue"){ |n, b| (n - b).abs == 1 }
problem.constrain("blend", "water"){ |b, w| (b - w).abs == 1 }
result = problem.solve

table = result.each_slice(5).map do |e|
  e.sort_by!{ |k, v| v }
  e.map{ |k, v| k }
end

require "terminal-table"

ttable = Terminal::Table.new do |tt|
  tt << [1, 2, 3, 4, 5]
  table.each do |t|
    tt << :separator
    tt << t
  end
end

puts ttable

Gemfile

source "https://rubygems.org"

gem "csp-solver"
gem "terminal-table"

question

The Brit lives in a red house.
The Swede keeps dogs as pets.
The Dane drinks tea.
The green house is on the left of the white, next to it.
The green house owner drinks coffee.
The person who smokes Pall Mall rears birds.
The owner of the yellow house smokes Dunhill.
The man living in the house right in the center drinks milk.
The Norwegian lives in the first house.
The man who smokes blend lives next to the one who keeps cats.
The man who keeps horses lives next to the man who smokes Dunhill.
The owner who smokes Blue Master drinks beer.
The German smokes Prince.
The Norwegian lives next to the blue house.
The man who smokes blend has a neighour who drinks water.

Who keeps fish?

result

+-----------+--------+---------+--------+---------+
| 1         | 2      | 3       | 4      | 5       |
+-----------+--------+---------+--------+---------+
| yellow    | blue   | red     | green  | white   |
+-----------+--------+---------+--------+---------+
| norwegian | danish | british | german | swedish |
+-----------+--------+---------+--------+---------+
| water     | tea    | milk    | coffee | beer    |
+-----------+--------+---------+--------+---------+
| dunhill   | blend  | pall    | prince | master  |
+-----------+--------+---------+--------+---------+
| cat       | horse  | bird    | fish   | dog     |
+-----------+--------+---------+--------+---------+

real	0m0.088s
user	0m0.084s
sys	0m0.000s